Allow to login with CACs that are issued globally

Users using smartcards with SAN extensions should be able to login
into gitlab, on the following two scenarios:

* The user certificate only has one email entry in the SAN extensions
  and it should be used to login into gitlab.
* The user certificate has multiple email entries and should only use
  the one that match the URI as described here
  https://docs.gitlab.com/ee/administration/auth/smartcard.html#authentication-against-a-local-database-with-x509-certificates-and-san-extensions-premium-only

doc/administration/auth/smartcard.md

@@ -51,10 +51,14 @@ This is an experimental feature. Smartcard authentication against local database
 change or be removed completely in future releases.
 
 To use a smartcard with an X.509 certificate to authenticate against a local
-database with GitLab, at least one of the `subjectAltName` (SAN) extensions
-need to define the user identity (`email`) within the GitLab instance (`URI`).
-
-`URI`: needs to match `Gitlab.config.host.gitlab`.
+database with GitLab, in:
+
+- GitLab 12.4 and later, at least one of the `subjectAltName` (SAN) extensions
+  need to define the user identity (`email`) within the GitLab instance (`URI`).
+  `URI`: needs to match `Gitlab.config.host.gitlab`.
+- From [GitLab 12.5](https://gitlab.com/gitlab-org/gitlab/issues/33907),
+  if your certificate contains only **one** SAN email entry, you don't need to
+  add or modify it to match the `email` with the `URI`.
 
 For example:
 

ee/changelogs/unreleased/33907-smartcard-flex-single-san-email-entries.yml

+---
+title: Allow to login with Smartcard certificates using SAN extensions that only defines
+  one global email identity
+merge_request: 20052
+author:
+type: changed

ee/lib/gitlab/auth/smartcard/san_extension.rb

@@ -27,7 +27,14 @@ module Gitlab
         end
 
         def email_identity
-          alternate_emails.find { |name| gitlab_host?(name[URI_TAG]) }&.fetch(EMAIL_TAG, nil)
+          email_entry =
+            if alternate_emails.size == 1
+              alternate_emails.first
+            else
+              alternate_emails.find { |name| gitlab_host?(name[URI_TAG]) }
+            end
+
+          email_entry&.fetch(EMAIL_TAG, nil)
         end
 
         def alternate_emails

ee/spec/lib/gitlab/auth/smartcard/san_extension_spec.rb

@@ -5,7 +5,7 @@ require 'spec_helper'
 describe Gitlab::Auth::Smartcard::SANExtension do
   let(:fqdn) { 'gitlab.example.com' }
   let(:extension_factory) { OpenSSL::X509::ExtensionFactory.new(nil, cert) }
-  let(:san_extension) { described_class.new(cert, fqdn)}
+  let(:san_extension) { described_class.new(cert, fqdn) }
 
   let(:cert) do
     key = OpenSSL::PKey::RSA.new 2048
@@ -69,42 +69,51 @@ describe Gitlab::Auth::Smartcard::SANExtension do
   end
 
   describe '#email_identity' do
-    let(:san_email) { 'newemail@some.domain' }
-    let(:san_uri) { "https://#{fqdn}" }
+    let(:san_single_email) { 'singleEntryEmail@some.domain' }
 
     before do
       allow(Gitlab.config.gitlab).to receive(:host).and_return(fqdn)
+      add_san_entry "email:#{san_single_email}"
     end
 
     subject { san_extension.email_identity }
 
-    describe 'alternate name email for GitLab defined in the certificate' do
+    it { is_expected.to eq san_single_email }
+
+    context 'multiple email identity SAN entries' do
+      let(:san_email) { 'newemail@some.domain' }
+      let(:san_uri) { 'not.yourdomain.com' }
+
       before do
         add_san_entry "email:#{san_email},URI:#{san_uri}"
       end
 
+      describe 'alternate name email for GitLab defined in the certificate' do
+        let(:san_uri) { "https://#{fqdn}" }
+
         it { is_expected.to eq san_email }
 
-      describe 'inappropriate URI format' do
+        context 'inappropriate URI format' do
           let(:san_uri) { 'an invalid uri' }
 
           it { is_expected.to be_nil }
         end
       end
 
-    describe 'no alternate name defined to use with GitLab' do
+      context 'no alternate name defined to use with GitLab' do
         it { is_expected.to be_nil }
       end
 
       context 'when the host is partially matched to the URI' do
-      let(:forged_uri) { "https://#{fqdn}.anotherdomain.com" }
-      let(:forged_identity) { 'hacker@email.com' }
+        let(:uri) { "https://#{fqdn}.anotherdomain.com" }
+        let(:identity) { 'user@email.com' }
 
         before do
-        add_san_entry "email:#{forged_identity},URI:#{forged_uri}"
+          add_san_entry "email:#{identity},URI:#{uri}"
         end
 
         it { is_expected.to be_nil }
       end
     end
+  end
 end